sediment. In some portions of the river system, sediment is overly abundant and for that reason can be considered a contaminant. In other places it is considered a natural resource in deficient supply.

At the scale of the entire Mississippi River, including its effects that extend into the northern Gulf of Mexico, nutrients and sediment are the two primary water quality problems. Nutrients are causing significant water quality problems within the Mississippi River itself and in the northern Gulf of Mexico. With regard to sediment, many areas of the upper Mississippi River main channel and its backwaters are experiencing excess sediment loads and deposition, while limited sediment replenishment is a crucial problem along the lower Mississippi River and into the northern Gulf of Mexico. Nutrients and sediments from nonpoint sources are the primary water quality problems focused on in this report. With respect to nutrients and sediments (and some toxic substances), water quality in the lower Mississippi River is determined largely by inputs in the upper Mississippi River basin, with different portions of the upper river basin having a dominant influence for particular constituents. For example, sediment loads are determined largely by the Missouri River contributions, and nutrient contributions are primarily from the upper Mississippi River.

In addition to nutrient and sediment issues, the Mississippi River has a variety of other water quality challenges. Toxic substances, including PCBs, metals, and pesticides, have important human health implications and are related primarily to legacy inputs. Their concentrations, fortunately, have been decreasing with time, in large part due to reductions in point source contributions as a result of the Clean Water Act. Similarly, the Clean Water Act has been useful in substantially reducing fecal coliform levels in the Mississippi River. The Clean Water Act was designed to remediate some of the impacts of human activities and has been effective in reducing many impacts attributable to point sources. Many of today’s water quality problems, however, are nonpoint in nature.

Whereas the Clean Water Act has been successful in reducing many point source pollution problems along the Mississippi River, it has not been as successful in reducing nonpoint source pollutants. Both the source and the scale of Mississippi River and Gulf of Mexico nonpoint source water quality problems pose significant Clean Water Act-related management challenges. The following chapters describe the Clean Water Act and discuss challenges in its administration to achieve its goals of attaining fishable and swimmable water quality and restoring the chemical, physical, and biological integrity of water resources as these goals apply to the Mississippi River.

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